Phosphorus Supply And Transformation In Mollisol Under Reduced Phosphate Rate And Plastic Film Mulching

Over application of phosphate?Pi?fertilizer results in an accumulation of phosphorus?P?in soils and low phosphate recovery rate of crops.In the early spring of Northeast China,maize seedling growth is susceptible to low temperature stress.Plastic film mulching can reduce soil water evaporation and increase soil temperature.The aim of this paper is to elucidate P supply and transformation in Mollisol under plastic film mulching at maize seedling stage and continous reducing Pi fertilizer rate.Field experiments were conducted in Harbin from 2013 to 2017 and in Gongzhuling in 2018.Maize growth,nutrient uptake and soil available P were measured.Chloroform fumigation-extraction was used to determine the soil microbial biomass P.Phospholipid fatty acids?PLFA?were used to characterize microbial community composition.P fractions in soil samples were observed using sequential chemical fractionation.Soil enzyme activities were detected using a microplate fluorimetric assay.Oxygen stable isotopes in phosphate(?¹⁸O_P)were analyzed in soil.Four treatments including NK,NPK,NPK+manure and NPK+straw were selected in long-term fertilization experiment in Mollisol.Organic P compositions were identified with 2D ³¹P-¹H Heteronuclear single quantum coherence?HSQC?nuclear magnetic resonance spectroscopy?NMR?.The main results were as follows:1.Plant P content and utilization.Compared with the conventional Pi rate of 75 kg P₂O₅/hm²,maize yield was not affected under continous reducing Pi rate?60 kg P₂O₅/hm²?in five experimental years.Partial productivity of Pi fertilizer increased.In the fourth year without Pi fertilization began to decrease,maize yield,dry matter weight,plant P content and accumulation began to decline.The treatment of plastic film mulching at maize seedling stage improved plant growth and increased shoot P content.2.Soil P forms.Compared with Pi fertilization treatment of 75 kg P₂O₅/hm²,available P concentrations in rhizosphere soil and non-rhizosphere soil without application of Pi fertilizer was lower and showed a decreased trend with the extending experimental time.The soil available P concentrations also decreased in the third year with 60 kg P₂O₅/hm² treatment.The treatment of plastic film mulching at maize seedling resulted in lower available P in non-rhizosphere soil than that in rhizosphere soil.Compared with soil sampled at seedling stage in 2013 maize,the content of Pi extracted by resin and NaHCO₃ in soil sampled at harvest stage in 2016 decreased without Pi fertilizer,60 kg P₂O₅/hm² and reduced Pi rate+film mulching treatment.The NaOH and 1mol/L HCl extracted Pi concentrations decreased with treatments of no Pi fertilizer and 60 kg P₂O₅/hm²+film mulching.More decline was observed in rhizosphere than non-rhizosphere with film mulching treatment.Compared with 60 kg P₂O₅/hm² treatment,the Pi concentrations extracted using resin,NaHCO₃ and 1mol/L HCl in soils at maize seedling stage was higher,moreover,the proportion of organic P extracted by NaHCO₃ and concentrated HCl in total soil organic P is lower.3.Enzyme activity and microbial community composition.Compared with the conventional Pi rate of 75 kg P₂O₅/hm²,the activity of acid phosphatase in rhizosphere was lower under no and reduced Pi fertilizer rate.The activities of acid phosphatase,alkaline phosphatase,?-xylosidase,total PLFAs and microbial biomass P increased under film mulching condition.The proportion of bacteria and fungi in non-rhizosphre soil decreased significantly and the proportion of actinomycetes increased.The proportion of actinomycetes in non-rhizosphre soil was significantly higher than that in rhizosphere under film mulching.Principal component analysis showed that Gram-negative bacteria?G^-?and fungi had higher contribution to the microbial community in the rhizosphere soil;the non-rhizosphere soil had more Gram-positive bacteria G⁺.Soil pH and available P were positively correlated with G⁺/G^-.4.Oxygen stable isotopes in phosphate(?¹⁸O_P).Compared with the no Pi fertilization,the?¹⁸O_P with Pi fertilizer application at Harbin site did not show significant changes and the?¹⁸O_P value of the Jilin site was higher in rhizosphere.Compared with reduced Pi fertilizer,the changes of?¹⁸O_P under film mulching at Harbin and Gongzhuling sites was not obvious.Using the equilibrium equation temperature between?¹⁸O_P and the surrounding water oxygen stable isotope(?¹⁸Ow),the temperature T=111.4-4.3(?¹⁸O_P–?¹⁸Ow),the calculated?¹⁸O_P value in soils with film mulching treatment was closer to the measured value,indicating that the Pi utilization efficiency by soil micro-organism might be high under film mulching condition.5.2D ³¹P-¹H HSQC NMR analysis.2D ³¹P-¹H HSQC NMR detected that the long-term application of NPK treatment of soil organic phosphorus compounds was less.Phosphoethanolamine and?-glycerophosphate in NPK+straw treatment were detected with 2D ³¹P-¹H HSQC NMR technique,which overlapped in 1D ³¹P NMR spectrum.More organic P compounds could be detected in NPK+manure application.In summary,Maize yield under reduced Pi fertilizer was not affected in experimental periods of five years.Maize yield,dry matter weight,P concentration and accumulation began to decrease without Pi fertilizer treatment in the fourth year.Resin and NaHCO₃ extracted Pi under no Pi input and continous reducing Pi rate decreased with the extending time.The plastic film mulching improved maize growth and incerased plant P accumulation,moreover,it enhanced soil microorganisms and enzyme activities at seedling stage.Stable NaOH and HCl extracted Pi,and organic P were prone to transform to available P forms under plastic film mulching.The analysis of?¹⁸O_P and 2D ³¹P-¹H HSQC NMR could provide new technologies for studying soil P composition and transformation.